(1) Field of the Invention
This invention relates to the casting of metals, particularly aluminum and aluminum alloys, by direct chill casting techniques. More particularly, the invention relates to the co-casting of metal layers by direct chill casting.
(2) Description of the Related Art
Metal ingots are commonly produced by direct chill (DC) casting of molten metals in which a molten metal is poured into a mold having an open upper end and (after start-up) an open lower end. The metal emerges from the lower end of the mold as a metal ingot that descends as the casting operation proceeds. In other cases, the casting takes place horizontally, but the procedure is essentially the same. Such casting techniques are particularly suited for the casting of aluminum and aluminum alloys, but are suitable for the casting of other metals as well.
Casting techniques of this kind are discussed extensively in U.S. Pat. No. 6,260,602 to Wagstaff, which relates exclusively to the casting of monolithic ingots, i.e. ingots made of the same material throughout cast as a single layer or ingot. Apparatus and methods for casting layered structures by DC casting are disclosed, for example, in U.S. Pat. No. 6,705,384 to Kilmer et al., issued Mar. 16, 2004, and in U.S. Patent Publication No. 2005/0011630 A1 to Anderson et al., published on Jan. 20, 2005. The Kilmer et al. patent makes use of a metallic divider member suspended in a direct-chill mold. The divider member separates the mold into two chambers that may be supplied with different molten metals, and the member becomes part of the ingot as the molten metal freezes. Consequently, the divider member is continuously fed into the mold through the entry end as the casting operation progresses so that part of the divider member is always present in the mold to keep the molten metal pools separated from each other. In contrast, the Anderson et al. publication employs so-called sequential solidification which requires the casting of a first layer (e.g. a core ingot) and allowing it to cool to the extent that it forms a solid (or at least semi-solid) outer surface, and then, subsequently but in the same casting operation, casting one or more layers of other metal on the solidified surface of the first metal layer. This can be achieved by providing a cooled divider wall at the entrance of the mold to divide the mold entrance into two chambers for receiving feeds of different molten metals. The divider wall remains in place during the casting operation and does not become incorporated into the solidified ingot. The length of the divider wall (in the axial direction of the mold) is long enough to permit the first layer to form its solid shell before it comes into contact with molten metal forming additional layers. The disclosures of the Wagstaff, Kilmer et al. and Anderson et al. references are specifically incorporated herein by this reference.
Ingots produced by both of these co-casting techniques, i.e. the use of a continuously supplied divider member that becomes incorporated into the ingot, and the provision of a cooled divider wall, may suffer from certain disadvantages, especially when intended for subsequent rolling into sheet products, such as brazing strip. One problem is that a relatively thin coating layer formed on a thicker core ingot may be “wiped off” during rolling at the leading and trailing ends of the ingot (i.e. at the ingot head and butt ends), and also at the width sides of the ingot. These phenomena are referred to, respectively, as head-, butt- and edge-wiping, and involve a squeezing of the metal of the coating layer beyond the ends or sides of the ingot at the points where localized rolling pressures may be higher than those over the remainder of the ingot. Another problem is that, because the ingot is subjected to different cooling dynamics during the main stage of the casting operation than at the start and end of casting, so that the cooling ingot is subjected to different rates of contraction in these stages, the interface between the layers may become non-planar in the final cast ingot. This may cause differences of thickness of the coating layer after rolling.
There is therefore a need for improvements to casting apparatus and methods of this kind.